Help with water chemistry... save my fish!

Hi! I have a 7 inch common goldfish in a 20 gallon tank that has been perfectly cycled for 2 1/2 years. I do weekly 50% water changes and he has been happy and healthy. We went on vacation for five days and left the neighbors in charge of feeding our fish. When we came back home, the tank was in full algae bloom and the fish was lethargic on the bottom of the tank.

At that point the nitrates we're out of control but ammonia and nitrites were zero. I did a 50% water change, added freshwater aquarium salt for stress, and used prime conditioner. The nitrates came down a bit, but not much so before I went to bed I did another 50% water change. He started swimming around and looking better.

This morning he looked really bad, barely alive, so before I tested the water, I just did a 50% water change to try and save him. He seemed a bit better, but now he's just kind of sitting on the bottom and not moving much at all. I just tested his water again and his nitrates were much lower (10-20) but now his ammonia is 1.0 and is nitrites are between .25 and .50. Ph has been fine.

Should I wait it out, or do a 4th water change in less than 24 hours? I don't know what happened while we were out of town but I really don't want to lose this fish. He's become part of the family!

I would guess they put way too much food. The excess may be sitting somewhere in the tank (in the gravel maybe) and rotting causing an ammonia spike.
I would do the water changes, clean the gravel out and stop feeding for now

20 gallons for a 7 inch comet, which will certainly grow larger if not already stunted? Wow. I'm shocked to stumble across such advice.
I should clarify that I do agree that 20 gallons per fancy goldfish is just fine, however comets grow too large and need much more swimming room.

Comets do not grow larger than fancies. They get longer, and retain their slender body and flexibility. My 12" comets can easily turn around in the 6" wide minitank that I use to take their pictures. Fish have three dimensions. Fancies are both taller and wider than comets. A 7" comet probably has a standard length (not including tail) of 4". It has roughly the same mass as an "average" fancy with 2" standard length.

How many comets and commons do you have? What size tank do you keep them in? What have you observed with your fish that lead to your statements?

Comets do not grow larger than fancies. They get longer, and retain their slender body and flexibility. My 12" comets can easily turn around in the 6" wide minitank that I use to take their pictures. Fish have three dimensions. Fancies are both taller and wider than comets. A 7" comet probably has a standard length (not including tail) of 4". It has roughly the same mass as an "average" fancy with 2" standard length.

How many comets and commons do you have? What size tank do you keep them in? What have you observed with your fish that lead to your statements?

I'm sure your foot long comet "can" turn around in your 6" mini tank, and I agree that fancies do have roughly the same mass as comets. However my main point is that from what I've read and witnessed from fellow goldfish keepers (no personal experience), comets need more swimming space. They can survive in a cramped, clean 20 gallon, but why should they have to? They are much better suited as pond fish IMO.
This quote from WetWebMedia.com says it best:
"The comet is a long-tailed version of the common goldfish, and widely known for being the greyhound of the goldfish family, being capable of astonishing bursts of speed. Of all the goldfish traded, this is the one species that really does thrive in a pond environment where it has space to swim."

I currently have one 3" oranda, two 2" ryukins and a 3" watonai in a 75 gallon tank.